Field of the Invention
The present invention relates to an exposure method, an exposure apparatus, and an article manufacturing method.
Description of the Related Art
Various devices, for example, a semiconductor device and a flat panel display (liquid crystal display device) are manufactured through a photolithography process. The photolithography process includes an exposure process in which a pattern of an original referred to as a “mask” or a “reticle” is projected and exposed to a substrate such as a glass plate or a wafer coated with photosensitizer referred to as “resist”. In recent years, improvement of the focus accuracy, which indicates accuracy of matching a substrate surface with an imaging plane of a projection optical system, and improvement of alignment accuracy, which indicates accuracy of accurately superposing pattern layers formed through a plurality of processes, and the like have been important in order to improve exposure accuracy so as to meet a demand for further miniaturization of the pattern.
In this context, when an exposure light is continuously irradiated to the projection optical system for a long time, heat is generated by absorbing a part of the exposure energy, and as a result, imaging performance (focus, magnification, distortion, astigmatism, wave front aberration, etc.) changes, and focus and alignment errors that cannot be ignored may occur. In contrast, an exposure method that successfully adjusts the change of the imaging performance even when an illumination condition of the exposure light changes and the heat-generation distribution of a lens in the projection optical system changes, has been proposed. Japanese Patent No. 2828226 discloses an exposure method in which a correction coefficient of the imaging performance corresponding to a light source image distribution state of the illumination light is stored, corresponding correction information is read out when the light source image distribution state is changed, and the correction is performed on the basis of the information. However, in the exposure method disclosed in the Japanese Patent No. 2828226, immediately after the change of the illumination condition, the temperature distribution occurring due to the illumination conditions before the change remains in the projection optical system. Thus, there may be cases where an offset in accordance with the amount of influence of the absorption of the illumination light before the change occurs in the imaging performance under the illumination condition after the change. Accordingly, Japanese Patent No. 3395280 discloses an exposure method in which the occurrence of the offset of the imaging performance immediately after the change of the illumination condition is eliminated by correcting the correction amount of the imaging performance on the basis of an accumulated energy amount according to the illumination condition before the change.
In this context, when the exposure is continued under the illumination condition after the change, the temperature distribution in the lens in the vicinity of a pupil plane of the projection optical system becomes a transient state in which the influence under the illumination condition before the change and the influence under the illumination condition after the change are overlapped. In contrast, in the exposure method disclosed in the Japanese Patent No. 3395280, because the correction amount of the imaging performance is corrected by focusing only on the offset amount immediately after the change of the illumination condition, it is difficult to accurately calculate the change amount in such a transient state.
In contrast, for example, there is a method for performing exposure while controlling the imaging performance of the projection optical system under a new illumination condition, after stopping the exposure until influence of the change amount of the imaging performance becomes small when the illumination condition is changed corresponding to the original or its pattern. Here, “until influence of the change amount of the imaging performance becomes small” refers to the point at which the change amount of the imaging performance, which is due to absorption of the illumination light of the projection optical system under the illumination condition before the change, becomes a predetermined allowable value or less. This can also be called the point in time at which the influence on the imaging performance of the energy amount accumulated in the projection optical system before the change becomes negligible. According to this method, exposure is not performed under the transient state when the illumination condition is changed, and thus, the imaging performance of the projection optical system can be strictly controlled for each illumination condition. However, in this method, because it is necessary to stop the exposure each time the illumination condition and the pattern of the original (that is, illumination distribution in the pupil plane of the projection optical system (light source image distribution) changes)), throughput of the exposure apparatus decreases. Additionally, as another method, in the transient state after the change of the illumination condition, it is assumed that exposure is performed while successively measuring the imaging performance of the projection optical system by using a reference mark on a stage that holds the substrate, and correcting the imaging performance as needed based on this measurement result. However, it is necessary to perform the measurement of the imaging performance after temporarily stopping the exposure also in this method, and a decrease of the throughput cannot be avoided.